# a = [1, 2, 3, 4]
# b = [5, 6, 7]
# print(a + b)
# print(b + a)


a = [1, 2, 3, 4]
b = [5, 6, 7]
#a.extend(b) 则是直接把b的内容拼接到了a的后面   ——>高效
a += b  #  a += b <==> a = a + b          ——>低效
print(a)
print(b)
